A Coupled Cluster Approach to the Electron Correlation Problem Using a Correlated Reference State

The Coupled Cluster (CC) formalism,1,2 starting from a single reference function, has been eminently successful for treating electron correlation for closed shell systems.3 Several generalizations to encompass multireference functions, leading to CC theories for open-shells, have been suggested in recent years4 in the framework of effective hamiltonians, which have offered interesting possiblities of computing directly energy differences of spectroscopic interest. Although originally designed for complete model spaces,5 it has been found necessary to develop the open-shell CC theory for incomplete model spaces4,6 since otherwise there is often the danger of the intruder problem. It appears that the intruders can to a large extent be avoided for spectroscopic energy differences if one works with incomplete model spaces, but they remain a real menace for computing potential energy surfaces, since there are different intruders for the different regions of the surface.